Radiant electric beaters are very well known, provided underneath and in contact with a cooking plate, particularly of glass-ceramic material, It is common practice to provide such heaters with thermal sensors of electromechanical or electronic form, the purpose of which is to limit maximum temperature of the upper surface of the cooking plate.
WO 95/16334 describes the use of a one- or two-dimensional thermoelectrical sensor based on radiation from a vitroceramic surface to control the temperature of the vitroceramic surface, the sensor possibly being shielded from direct radiation from the heating elements.
U.S. Pat. No. 4,103,275 describes a means for measuring resistance for a resistance thermometer consisting of an insulating former as a carrier and a thin platinum layer as resistance material, the carrier for the platinum layer being made of a material having a greater thermal coefficient of expansion than platinum over the range between 0 degrees Celsius and 1000 degrees Celsius.
In current technology, a temperature sensing probe is located in a space between a heating element and the underside of the cooking plate.
A disadvantage of such an arrangement is that the temperature attained by the sensing probe is significantly influenced by direct radiation from the heating element and does not accurately reflect the temperature of the upper surface of the cooking plate. The probe temperature can typically be 100 to 200 degrees Celsius higher than the corresponding temperature of the upper surface of the cooking plate, As a result, there are two temperature gradients between the sensing probe and the upper surface of the cooking plate, namely one temperature gradient between the sensing probe and the underside of the cooking plate and another temperature gradient between the underside of the cooking plate and its upper surface. These temperature gradients may vary as a result of, for example, changes in heater power density, heater temperature profile, and thermal loading by a selected cooking vessel located on the upper surface of the cooking plate. Such a cooking vessel affects the temperature of the upper surface of the cooking plate.
A temperature sensor used in such heaters is arranged to de-energise the heating element at a preset temperature value. Such preset temperature value is a compromise value to maintain acceptable maximum temperatures of the cooking plate under the requirements of abnormal load conditions (for example: no cooking vessel load; boil dry; offset cooking vessel on cooking plate; cooking vessel with a concave base, brought to a boil condition), while minimising the probability of de-energising the heating element under bring-to-boil conditions in respect of a load in the form of a cooking vessel located on the cooking plate. Repeated switching of the heating element under the latter conditions is undesirable, since the time to boil is increased.
When electronic temperature sensors are employed, the probability of such undesirable switching of the heating element occurring may be significantly reduced by incorporating an intelligent control profile within a dedicated ‘fast boil’ control setting. This necessitates use of intelligent, usually digital, microprocessor controllers, which are expensive.
The tolerance range of the preset temperature value of the temperature sensor is critical, as it compounds the aforementioned variables. Electromechanical temperature sensors yield a tolerance range of typically 50 to 60 degrees Celsius as a result of constraints imposed by materials, design and manufacturing technology. Currently available electronic temperature sensors exhibit much lower tolerance ranges, but these devices, together with their required control circuits, cost significantly more than electromechanical temperature sensor systems.
Furthermore, due to the aforementioned variables and temperature gradients, an electronic temperature sensor, as previously described, is only useful as a maximum temperature control device, such that it de-energises the heating element at a predetermined temperature value. Such an electronic temperature sensor is unable to support a control system that may control the temperature of the cooking plate in accordance with a required cooking duty cycle, involving ‘closed loop’ control temperature regulation, Current temperature regulation systems for cooking appliances having glass-ceramic cooking plates are ‘open loop’ in nature. Such temperature regulation systems cannot account for variations in cooking vessel material and geometry, cooking vessel mass, mass and thermal capacity of a food item in a cooking vessel, and most importantly, change in temperature gradient as the cooking vessel and contents heat up, accompanied by evaporation of water. Constant adjustment of the heater is required by a user, especially at low settings.
Furthermore, it is anticipated that the maximum operating temperatures for glass-ceramic cooking plates will be increased in the near future by as much as 40 degrees Celsius, as a result of materials and process development. The objective of this increased temperature is to provide opportunity for higher temperatures to be reached before switching of the heating element occurs, thereby reducing the probability of de-energising of the heater during a bring-to-boil cycle. Currently available temperature sensors may require further development in order to withstand the resulting higher maximum temperatures encountered during service, because of the constraints imposed by existing sensor element and enclosure materials. This could lead to increased cost of the sensor system.